Five-year-old Harmony Fielden may be the only child in the world with a bone disease that is baffling the medical community.

Her mother Samantha Fielden said Harmony had been part of a 12-month research program conducted by doctors at Westmead Children's Hospital, who had ruled out several conditions, but were yet to identify the exact problem.

"They're really trying to find what is wrong with her and they know it's her bone structure," Ms Fielden said.

"The doctors just rang me and said 'Sam, we don't know what she's got, we've just run all of these tests, she could be the only child in the world with it or she could have created it on her own'."

It's been misdiagnosed a lot of times, so now the doctors just say they don't know what it is. We're not 100 per cent what to call it.

Samantha Fielden, mother

Ms Fielden said she first noticed something "wrong" with Harmony when she was a baby.

"When she was about six months or seven months old, you could notice there was something different," she said.

"At 17 months, we could tell there was something seriously wrong with her.

"You could tell by the bowed-ness of her legs, and so we went to see a doctor.

"The doctor looked at her legs and thought we should go and get some x-rays, and while we were waiting to have those done, we went to see a physio as well and they said 'it's normal, it's nothing'.

"Then we got the x-rays back and they discovered it's not normal; there's something severely wrong with her bones."

Ms Fielden said Harmony had endured numerous tests and misdiagnoses throughout her life.

"We got a report from the x-ray place saying they thought it was metaphyseal dysplasia disease, but it wasn't," she said.

"It's been misdiagnosed a lot of times, so now the doctors just say they don't know what it is.

"We're not a 100 per cent what to call it."

Holes in bones, gaps in hips, scoliosis in spine

Ms Fielden said the best way to describe her daughter's 'problem' was to list what the x-rays had shown.

She said Harmony had gaps where her hip joints should have been, holes in some bones, certain bones shorter than others, and scoliosis in her spine.

Now, when people ask us, I say she's been able to walk, crawl, talk and so I feel lucky to have her.

Samantha Fielden, mother

Harmony is also small for her age.

"They've thought dwarfism might have come into it as well," Ms Fielden said.

"My three-year-old is a fair size bigger than Harmony."

Ms Fielden said Harmony had experienced significant pain up until an operation on her hips and legs in 2014.

More surgery, focusing on her knees, is scheduled at Westmead Children's Hospital on March 9.

"In her hips, because she had no ball and hip joint, they decided to fuse the bones together so she has a join there," Ms Fielden said.

"In her knees, you have growth plates and they bolted bolts above the knee and below the knee into her growth plates to try to make the outside [of her] legs stop growing but make the inside [of the] leg grow.

"The hip one, with the plates and the pins has worked well, but in her knees it hasn't worked, so that's why she needs more surgery in March."

Family deals with uncertainty

Ms Fielden said the family had gradually learned to deal with the uncertainty surrounding Harmony's condition.

"I think the hardest was when we first found out; they said this is extremely rare, and when they diagnosed her with metaphyseal dysplasia I was shattered, because I read what it could do to her," she said.

I'm going to beep the beeper and get an ice-cream while Mummy is sleeping.

Harmony Fielden, aged 5

"Then they diagnosed her with something else and then another thing and another thing, so in the end I got used to it.

"Now, when people ask us, I say she's been able to walk, crawl, talk and so I feel lucky to have her.

"If we find a diagnosis for her, yes, but if not, it's not going to change much."

However, Ms Fielden said the constant travel to Sydney from their home town of Grafton, in northern New South Wales, had taken a financial and emotional toll on the family.